One difficulty with utilizing fuel cells to power the propulsion system of electric vehicles is the requirement that such vehicles be operable at temperatures below that at which water will freeze (below freezing). Freezing may cause mechanical damage as a consequence of the expansion of ice, and presents problems due to the inseparability of water and the fuel cell processes. Heretofore, various methods of initiating operation of a fuel cell, preparatory to the operation of an electric vehicle, have concentrated on providing heat, either by reaction or combustion of fuel, or by means of battery power, to various water and other coolant conduits and reservoirs. Other efforts are directed toward processes designed to accelerate the rate at which a fuel cell stack will heat up to above-freezing temperatures, as a consequence of its own operation. In U.S. Pat. No. 5,798,186, a proton exchange membrane (PEM), fuel cell is warmed up simply by connecting a load across it while stochiometric fuel and oxidant are supplied to the stack. In one experiment, with the fuel, oxidant and coolant water passages all having been purged of water upon previous shutdown of the stack, application of hydrogen and air at room temperature caused a temperature in the core of a ten cell stack to advance from −11° C. to 0° C. in about one minute. A four cell stack, in which only the reactant channels (and not the coolant channel) were purged upon previous shut down, required five minutes, after circulation of hydrogen and oxygen began and a 50 amp load was connected, to increase from −19° C. to 0° C. Coolant was not circulated until about 23 minutes after startup. In a four cell stack in which none of the channels were purged at the prior shut down, flow of warm hydrogen did not begin to occur until after four minutes, and 12 minutes expired between startup at −23° C. and reaching 0° C. within the core of a four cell stack. In U.S. Pat. No. 6,329,089, individual fuel cells at −5° C. started with room temperature hydrogen and air reached 0.5 amps per cm2in five minutes. With a short circuit load, a seven cell stack with a core temperature of −15° C. reached 0.5 amps per cm2 nine minutes after prolonged short circuiting of the stack output. Performance of other experiments were less satisfactory.
For use in vehicles, such as automobiles, an electric propulsion system must be operating in less than one minute, preferably less than one-half minute, after initiating startup. In commonly owned, copending U.S. patent application Ser. No. 10/187,547, filed Jul. 1, 2002 now abandoned, a vehicle propulsion system is started up in a condition in which at least a portion of the fuel cell stack is below the freezing temperature of water, and the entire vehicle may be in an ambient environment which is below the freezing temperature of water. Substantially all of the water in the hydrophilic support plates and the reactant gas flow fields and coolant channels is removed, which may be achieved in accordance with a procedure disclosed either in U.S. patent application Ser. No. 09/826,739, filed Apr. 5, 2001 now U.S. Pat. No. 6,596,426, or Ser. No. 09/826,739, filed Apr. 5, 2001. Thus, there may likely be ice in the PEM, and the anode and cathode catalyst layers, as well as within the pores of the water transport plates. However, it should be borne in mind that the water transport plates themselves can hold ice within the pores without doing damage to the water transport plates, as in the case for the PEM. Furthermore, the substrates are sufficiently close to the PEM itself that the temperature of the water that is transferred into the substrates will very nearly follow the temperature of the PEM, rather than the temperature of the reactant gases themselves. Thus, even though the water transport plates have ice in the pores thereof, product water will not freeze in the substrates.
In said copending application, a PEM fuel cell stack at subfreezing temperature is connected to a vehicle propulsion system within a few seconds, or as soon as the stack provides open circuit voltage. The fuel cell stack is started with more than a stochiometric flow of fuel and at least three times to five times stochiometric flow of oxidant, which may be at subfreezing temperatures, whereby to prolong operation without localized heating, thereby permitting the vehicle to be used during the time that the apparatus and fluids are being heated to suitable, operational temperatures. That system takes advantage of reactants which are at the same sub-freezing ambient temperature as the fuel cell stack itself, since that prolongs the onset of localized overheating. In systems in which porous water transport plates are used for water management, heating of the water stored as ice in the pores of the water transport plates, by heating up the mass of the stack, heating up of that water, the heat of fusion as the ice melts, and evaporative cooling of some of that water, all prolong the period of time at which the vehicle can be operated with power from the fuel cell stack, without circulating coolant, before there is impermissible local heating within the fuel cell. Such time is about 15–20 minutes in a typical case.